1. Field of the Invention
The present invention relates to the use of water or some other aqueous liquid as the light conducting core medium of an elongated, rigid, small diameter vessel employed for light transmission. More particularly, this invention is directed to rigid tubular light guides suitable for spectrophotometric applications, and to methods of and apparatus for measuring the absorption of light in small volume aqueous fluid samples with the use of such light guides. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention has applicability to the field of fiber optics. Liquid core fiber-optic wave guides, i.e., light guide fibers in the form of a capillary filled with a fluid which functions as the light transmitting core, have previously been proposed. For an example of such a prior liquid-core fiber-optic wave guide, reference may be had to U.S. Pat. No. 3,894,788. Since light cannot be efficiently propagated through a fluid filled capillary unless the refractive index of the capillary is less than that of the core fluid, the wave guides of U.S. Pat. No. 3,894,788 use an organic fluid as the core liquid. These organic fluids are specially selected so as to have refractive indices which are greater than that of the particular material from which the capillary is fabricated in order to permit long distance propagation of light waves through the core liquid.
There has been a long standing desire to employ water or some other aqueous fluid in a liquid-core fiber-optic environment for the purpose of facilitating chemical analyses of aqueous solutions by light interactive processes such as light absorption, colorimetry and fluorescence. However, consistent with the teachings of U.S. Pat. No. 3,894,788, it has previously been universally believed that the low index of refraction of water and other aqueous liquids rendered it impossible to employ such materials as the light conducting core medium of a liquid-core, fiber-optic wave guide or the like.
A variety of techniques are available for use in the analysis of fluid samples. These techniques include optical methodology, particularly photometry and spectrophotometry, wherein the composition and concentration of dissolved substances are determined by measuring the absorption of light in a liquid which includes such substances. These optical analysis techniques are based on the fact that different substances will absorb light at different wave lengths. In the practice of these optical techniques, light absorption at discrete wave lengths or over a broad light spectrum, including ultraviolet, visible or infrared spectra, may be measured.
The need for instruments capable of the optical analysis of aqueous samples in the sub-milliliter volume range has grown in recent years. An important reason for this growing need is the fact that protein and DNA samples are usually in small volume aqueous samples. For example, it is often difficult to obtain large amounts of animal, especially human, tissue samples which must be analyzed. It is also costly to synthesize or purify protein, enzyme, antibody and DNA samples in large amounts.
Conventional absorption spectrometers are not sufficiently sensitive to analyze solutions prepared from the very small volume samples discussed above. For example, the approximate detection limit, defined as the lowest concentration that can be distinguished from background signal for double stranded DNA using absorption at a wave length of 260 nm is about 250 nanograms for a 0.5 ml, 10 mm light path length cuvette.
There have been efforts to reduce the requisite sample cuvette volume. Such efforts have often been characterized by a reduction in the light path length which, in turn, reduces instrument sensitivity. The smallest commercially available fluid sample cuvettes with 10 mm long light paths contain fluid volume in the 30 ul to 50 ul range. For a 5 ul volume cell, however, the path length would be limited to 0.5 mm and thus unsatisfactory for analysis.